Presently, a number of manufacturers manufacture and market radios for use in communications, such as digital cellular radios and the like. Typically, each manufacturer provides its own specifications for its products. Traditionally, the accuracy of these specifications has been measured using many separate, possibly indirect methods. Phase accuracy of the transmitted signal, for example, typically is indirectly determined by measuring spurious signals, phase noise, the modulation index, frequency settling speed, carrier frequency and data clock frequency. Further, amplitude measurements present special problems because the amplitude versus time profile must be synchronized to the data typically utilizing external equipment.
It has been proposed that a standardized mobile digital radio system be implemented throughout Europe. Such a radio system would require that all components, such as transmitters and receivers, be manufactured to standard specifications measured by a common method. A group known as the Group Speciale Mobile (GSM) has proposed a measurement technique to measure the accuracy of the modulation process of the transmitted signal. In the proposed measurement technique, a sampled measurement of the transmitted phase trajectory is obtained. This measurement is compared with an accurate estimate of the ideal phase trajectory to determine the phase difference between the transmitted signal and the ideal signal. The slope of the regression line of the phase difference thus determined provides an indication of the frequency error, and the regression line is subtracted from the phase difference to give the phase error. Utilization of a standard method, such as this, would simplify the testing and manufacture of radios. An individual manufacturer would then only need to insure that the standardized overall phase error specifications were met rather than several, interrelated specifications.
.pi./4 DQPSK is the modulation standard for the North American Cellular Digital Radio System. The standard was prepared by the Tr 45.3 Subcommittee of Electronic Industries Association/Telecommunications Industries Association (EIA/TIA). Included in the standard are specifications for the modulation accuracy of the transmitter. This standard includes specifications on the average frequency error and the error-vector magnitude of the transmitter. A method and apparatus for producing .pi./4 DQPSK modulation is disclosed in copending application Ser. No. 07/529,035 filed May 25, 1990.